1. Field of the Invention
The present invention relates to an optical element formed with a bonded pair of substrates, and a method for producing the optical element.
2. Related Background Art
In an optical element formed by bonding two substrates, a ridge optical waveguide can be formed by forming a ridge structure after thinning one of the substrates. For bonding these substrates, the direct bonding technique is known as a technique for firmly bonding these substrates without using an adhesive or the like. The direct bonding allows various materials such as glass, semiconductors, ferroelectrics, piezoelectric ceramics, etc. to be bonded with high precision, and therefore, the application of the same to optical elements has been highly expected. As an example of an optical element with use of a pair of directly bonded substrates (such a pair hereinafter sometimes is referred to as a direct-bond substrate) such as dielectric substrates, semiconductor substrates, and glass substrates, an optical waveguide-type element has been proposed. For instance, JP2574594 and JP06-222229A disclose a method for forming an optical waveguide by directly bonding lithium niobate or lithium tantalate as a ferroelectric crystal substrate with a substrate of the same type or a glass substrate.
Further, several proposals have been made regarding an optical element formed by bonding two substrates with a thin film interposed therebetween. In an optical element in which two substrates are used and one of them functions as a waveguide layer, the substrate functioning as the waveguide layer is required to have a higher refractive index. Therefore, a thin film having a lower refractive index than that of the waveguide layer is provided between the substrates, whereby light is guided irrespective of the refractive indices of the substrates. For instance, JP2574594 and JP06-222229A mentioned above disclose the use of SiO2 or SiN as a material for the thin film. Further, JP2574606 discloses the use of low-melting glass as the thin film material. JP06-289347A discloses the use of a metal oxide or the like as the thin film material.
As described above, the optical element formed with substrates of the same type having equal refractive indices without a thin film layer interposed therebetween cannot be used as an optical waveguide. Further, in the case where two substrates having different refractive indices are bonded directly, as in the case where a lithium niobate substrate and a Mg-doped lithium niobate substrate are bonded directly, it is impossible to form an optical waveguide in a substrate having the lower refractive index.
By providing a thin film between two substrates, the foregoing problems can be solved. However, it is difficult to provide a thin film between two substrates. As described in JP2574594 and JP06-222229A, it is difficult to control a surface roughness of a thin film layer in the case where SiO2, for instance, is used for forming a thin film, and a thin film formed by sputtering or vapor deposition has a significant roughness on its surface. A film with such a surface roughness is not suitable for direct bonding. The surface roughness can be reduced by, for instance, forming a thin film using a CVD (chemical vapor deposition) device, but the CVD device is expensive and bulky. Furthermore, the contact between a thin film and a substrate and the bond strength have a non-homogeneous distribution depending on conditions for the thin film formation, and when a bonded substrate pair is subjected to machining, a sufficient strength against the machining cannot be achieved.
Additionally, as shown in JP2574606, in the case where a low-melting glass is used for forming a thin film layer, for instance, a low-melting glass material is applied in a paste form over a substrate, bonded with another substrate, and subsequently baked. Therefore, it is difficult to control the film thickness so as to achieve a uniform thickness. Further, a technique described in JP06-289347A lacks practical utility, since metal oxide materials to be used for forming a thin film are not disclosed specifically in the publication.
Furthermore, for controlling a height of an optical waveguide and uniformity of the height thereof in forming an optical waveguide in a direct-bond substrate, it is important to determine a thickness of a substrate in which the optical waveguide is formed, among the substrates directly bonded, and to determine uniformity of the foregoing thickness. However, generally, it is difficult to determine optically the thickness uniformity of the optical waveguide with respect to a substrate where the waveguide is formed, and the control relies on the determination of a thickness of the entirety of the direct-bond substrate. Therefore, there is a drawback of insufficient thickness uniformity of the optical waveguide.